Cryptomatte & Render Layers: is it possible to recover Cryptomatte from a .PNG sequence?
Unfortunately there is no way to recover the Cryptomatte render passes, since .png isn't capable of storing them. For an explanation of the Cryptomatte format you can take a look at this answer. You will have to render it again and store the result in a Multi-layer OpenEXR.


  Why is it, every time I close Blender, my Render Layer in the
  compositor seems empty, losing the Cryptomatte pass?


Blender caches the last rendered frame and its render passes for compositing, they're not stored in the .blend file though and therefore not persistently written to your hard drive. Once you close Blender that data is gone.